tcp: annotate data-races around tp->keepalive_time

do_tcp_getsockopt() reads tp->keepalive_time while another cpu
might change its value.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230719212857.3943972-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

include/net/tcp.h

@@ -1517,9 +1517,12 @@ static inline int keepalive_intvl_when(const struct tcp_sock *tp)
 static inline int keepalive_time_when(const struct tcp_sock *tp)
 {
 	struct net *net = sock_net((struct sock *)tp);
+	int val;
 
-	return tp->keepalive_time ? :
-		READ_ONCE(net->ipv4.sysctl_tcp_keepalive_time);
+	/* Paired with WRITE_ONCE() in tcp_sock_set_keepidle_locked() */
+	val = READ_ONCE(tp->keepalive_time);
+
+	return val ? : READ_ONCE(net->ipv4.sysctl_tcp_keepalive_time);
 }
 
 static inline int keepalive_probes(const struct tcp_sock *tp)

net/ipv4/tcp.c

@@ -3312,7 +3312,8 @@ int tcp_sock_set_keepidle_locked(struct sock *sk, int val)
 	if (val < 1 || val > MAX_TCP_KEEPIDLE)
 		return -EINVAL;
 
-	tp->keepalive_time = val * HZ;
+	/* Paired with WRITE_ONCE() in keepalive_time_when() */
+	WRITE_ONCE(tp->keepalive_time, val * HZ);
 	if (sock_flag(sk, SOCK_KEEPOPEN) &&
 	    !((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))) {
 		u32 elapsed = keepalive_time_elapsed(tp);